ReentrantLock和ReentrantReadWriteLock类的构造器都含有一个布尔参数fair，它允许你控制这两个类的行为。默认fair值为false，它称为非公平模式（Non-Fair Mode）。在非公平模式下，当有很多线程在等待锁（ReentrantLock和ReentrantReadWriteLock）时，锁将选择它们当中的一个来访问临界区，这个选择是没有任何约束的。如果fair值是true，则称为公平模式（Fair Mode）。在公平模式下，当有很多线程在等待锁（ReentrantLock和ReentrantReadWriteLock）时，锁将选择它们中的一个来访问临界区，而且选择的是等待时间最长的。这两种模式只适用于lock()和unlock()方法。而Lock接口的tryLock()方法没有将线程置于休眠，fair属性并不影响这个方法。
下面我们将修改“使用锁实现同步”当中的范例来使用这个属性，并观察公平模式和非公平模式之间的区别。
1. 创建一个打印队列类PrintQueue。
import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;public class PrintQueue {//声明一个锁对象，并且用ReentrantLock类初始化private final Lock queueLock = new ReentrantLock(true);//实现打印方法public void printJob(Object doucument){queueLock.lock();Long duration = (long) (Math.random()*10000);System.out.println(Thread.currentThread().getName()+": PrintQueue: Printing a Job during "+(duration/1000)+" seconds");try {Thread.sleep(duration);} catch (InterruptedException e) {// TODO Auto-generated catch block e.printStackTrace();} finally {queueLock.unlock();}queueLock.lock();duration = (long) (Math.random()*10000);System.out.println(Thread.currentThread().getName()+": PrintQueue: Printing a Job during "+(duration/1000)+" seconds");try {Thread.sleep(duration);} catch (InterruptedException e) {// TODO Auto-generated catch block e.printStackTrace();} finally {queueLock.unlock();}}}
2. 创建打印工作类Job并且实现Runnable接口。
public class Job implements Runnable {private PrintQueue printQueue;public Job(PrintQueue printQueue){this.printQueue = printQueue;}@Overridepublic void run() {System.out.printf("%s: Going to print a document\n", Thread.currentThread().getName());printQueue.printJob(new Object());System.out.printf("%s: The Document has been printed\n", Thread.currentThread().getName());}}
3. 创建范例的主类Main
public class Main {public static void main(String[] args) {//创建一个共享的打印队列对象PrintQueue printQueue = new PrintQueue();//创建10个打印工作Job对象Thread threads[] = new Thread[10];for(int i=0;i<10;i++){threads[i] = new Thread(new Job(printQueue), "Thread"+i);}//启动10个线程try {for(int i=0;i<10;i++){threads[i].start();Thread.sleep(100);}} catch (InterruptedException e) {// TODO Auto-generated catch block e.printStackTrace();}} }
4. 程序运行结果如下
Thread0: Going to print a document Thread0: PrintQueue: Printing a Job during 4 seconds Thread3: Going to print a document Thread2: Going to print a document Thread1: Going to print a document Thread4: Going to print a document Thread5: Going to print a document Thread6: Going to print a document Thread7: Going to print a document Thread8: Going to print a document Thread9: Going to print a document Thread3: PrintQueue: Printing a Job during 9 seconds Thread2: PrintQueue: Printing a Job during 3 seconds Thread1: PrintQueue: Printing a Job during 7 seconds Thread4: PrintQueue: Printing a Job during 7 seconds Thread5: PrintQueue: Printing a Job during 5 seconds Thread6: PrintQueue: Printing a Job during 6 seconds Thread7: PrintQueue: Printing a Job during 0 seconds Thread8: PrintQueue: Printing a Job during 7 seconds Thread9: PrintQueue: Printing a Job during 5 seconds Thread0: PrintQueue: Printing a Job during 4 seconds Thread0: The Document has been printed Thread3: PrintQueue: Printing a Job during 5 seconds Thread3: The Document has been printed Thread2: PrintQueue: Printing a Job during 1 seconds Thread2: The Document has been printed Thread1: PrintQueue: Printing a Job during 9 seconds Thread1: The Document has been printed Thread4: PrintQueue: Printing a Job during 4 seconds Thread4: The Document has been printed Thread5: PrintQueue: Printing a Job during 2 seconds Thread5: The Document has been printed Thread6: PrintQueue: Printing a Job during 5 seconds Thread6: The Document has been printed Thread7: PrintQueue: Printing a Job during 9 seconds Thread7: The Document has been printed Thread8: PrintQueue: Printing a Job during 4 seconds Thread8: The Document has been printed Thread9: PrintQueue: Printing a Job during 6 seconds Thread9: The Document has been printed
